The present invention relates to triple-wall pipe extruders and particularly to a method and apparatus for simultaneously extruding two triple-wall pipes.
Single-wall plastic pipes made of solid PVC are in widespread use throughout the world. PVC pipe is lightweight, inexpensive and strong. PVC pipe has become a standard for many uses, such as, for example, for drains. In some cases, one single-wall pipe is extruded through a single die, while in other cases two single-wall pipes are simultaneously extruded by connecting a two channel Y-block between the extruder and a pair of dies.
For many applications, single-wall PVC pipe is somewhat over-designed, resulting in a waste of materials. In order to reduce waste, so-called triple-wall PVC pipes are used for many applications. A cross-sectional view of a typical triple-wall pipe 10 is shown in FIG. 4. Pipe 10 includes an inner wall 12 which is made of solid PVC plastic, a middle wall 14 which is made of PVC foam, and an outer wall 16 which is made of solid PVC plastic. Triple-wall PVC pipe is much lighter in weight than a single-wall pipe of the same thickness and, in addition, substantially less material is used. In some instances, a weight reduction as high as 29% has been achieved while still consistently passing the impact and strength requirements of ASTM-F-891.
The prior art includes many devices which are directed to extruding a triple-wall pipe, or more particularly, to dies which are designed to direct the output from a plurality of extruders into a format which will solidify into a triple-wall pipe. For example, U.S. Pat. No. 5,620,714 issued to Veen, discloses a distributor head for forming a tubular profile from one or more streams of extruded thermoplastic material. Veen discloses a die which includes one or more input channels for connection to one or more extruders and these channels divide one or more times before connecting to three concentric circular output channels at the opposite side of the die. Veen discloses use of three extruders, as shown in FIG. 1, including a first extruder 18 which provides thermoplastic material which forms the outer wall of a pipe, a second extruder 20 which provides thermoplastic material to form the middle or foam core wall of a pipe, and a third extruder 22 which provides thermoplastic material which forms the outer wall of a pipe. These extruders feed a die 24 which extrudes the thermoplastic material into concentric cylindrical walls which are then cooled within a cooling device 26 with the completed triple-wall pipe exiting the output opening 27 of cooling device 26.
A further prior art disclosure related to a die for shaping the output from a dual extruder system into a triple-walled pipe is shown in FIG. 2. This drawing is found in U.S. Pat. No. 4,846,648 issued to Spence et al., and entitled "Triple-wall Foam Co-Extrusion Apparatus." As shown in FIG. 2, Spence discloses a die apparatus 24 which includes a first extruder connection channel 28 and a second extruder connection channel 30. The first extruder connection channel 28 is designed for connection to a first extruder (not shown) in order to receive thermoplastic material, such as PVC, which is used to form the solid inner and solid outer wall of a triple-wall pipe, and the second extruder connection channel 30 is used to receive foam thermoplastic material from a second extruder (not shown) which is then fed through the die in order to form the middle wall or foam core of the triple-wall pipe.
In order to properly operate a single or a dual extruder system to form a triple-wall pipe it is desirable to create a die which does not constrict the flow of thermoplastic material, which delivers thermoplastic material at the proper rate to form an inner and outer wall and foam core of uniform thickness, and finally, a die which is well suited to the particular output capacity of the extruders to which it is connected. Experience has shown that extruders used to form the middle or foam core wall of a triple-wall pipe are typically not used to their full capacity due to the smaller amounts of thermoplastic material which is required to form the middle or foam core wall. This is particularly true with respect to the formation of smaller diameter pipes. Therefore it would be preferable to devise a method and apparatus for utilizing the full capacity of the extruders used to produce the foam thermoplastic material for the middle wall of a triple-wall pipe.
As previously discussed, the Spence die shown in FIG. 2 requires two extruders to produce one triple-wall pipe. Accordingly, four extruders are required to produce two triple-wall pipes at the same time. Providing a means to produce a greater amount of pipe in the same time frame with fewer extruders would increase the energy efficiency and lower the energy cost of the production line. The foam extruder would operate within a higher range of its capacity and there would be no heat loss from the additional tooling required for the four-extruder process. The invention would also save on floor space and require lower maintenance costs versus the four-extruder method.